Issue #2859663 by alexpott, marncz, dawehner: Ignore core/node_modules

example.gitignore

@@ -14,6 +14,10 @@
 # Core's dependencies are managed with Composer.
 vendor
 
+# Ignore node_modules folder created when installing core's JavaScript
+# dependencies.
+core/node_modules
+
 # Ignore configuration files that may contain sensitive information.
 sites/*/settings*.php
 sites/*/services*.yml